This invention relates to ground fault protection arrangements for electrical distribution systems, and is more particularly though not exclusively concerned with electrical service entrance boxes for domestic and industrial electrical installations of the kind including a means to interrupt the electrical supply in the event of fault conditions arising in the installation.
In the conventional service entrance box, protection against ground faults and circuit overloads is by way of fuses or alternative circuit breaking devices which respond to the passage of excessive current through main or branch circuits connected to the line conductors of the supply.
The disadvantage of this conventional arrangement is that only ground faults which allow a current in excess of the maximum permissible load in the main or branch circuit concerned will result in operation of the circuit breaking device. However, ground faults can easily occur which are of sufficiently high resistance to fail to overload the circuit concerned, although passing sufficient current to cause intense local heat generation. Such faults are a major cause of serious fires. Even if the ground fault itself involves a short circuit, the ground return system in the electrical installation, represented by a system of raceways, conduits and/or ground return wires, may include poor connections or other zones of high resistance which themselves can limit the current flowing as a result of a ground fault to a level insufficient to operate the circuit breaker device, whilst a fire hazard still exists due to excessive generation of heat at points within the ground return system. Unfortunately, defectively installed ground return systems which only provide a relatively high resistance return path are all too common.
A number of attempts have been made to overcome this problem. The most frequent approach has been to detect any imbalance between the current flowing in the line conductor and the neutral conductor in a circuit since if these currents are not equal, it follows that an alternative current path, normally through the ground return system, must exist. In one commonly used form, such a device consists of a differential transformer having primary windings in series with the line and neutral conductors, and a secondary winding controlling a circuit breaker. Such systems are relatively sophisticated, and cannot easily be incorporated in existing installations.
A further problem with both the above systems is that they are incapable of detecting faults occurring on the supply side of a service entry box, for example by reason of a short circuit or leakage between a line conductor and an entry conduit.
It would be desirable if an effective and simple system could be provided, which could be incorporated into existing installations, and which would reliably protect the entire installation against the existence of ground faults which, whilst insufficient to operate conventional circuit breaking arrangements protecting individual branch circuits, nevertheless are of sufficient magnitude to present a fire hazard.